The integrity of DNA is constantly challenged by damage induced from environmental agents and normal metabolism. The purpose of this proposal is to reveal the molecular details underlying the Rad51 enzymes role in DNA repair and DNA recombination by investigating the central hypothesis that specific structural conformational states govern its reactivity. This will be accomplished by completing the following specific aims: 1) To test the hypothesis that ATP hydrolysis promotes conformational changes that alter Rad51 assembly and activities by X- ray crystallographic structure determination of Rad51 protein in the presence of ATP and ADP. 2) To test the hypothesis that DNA bases are exposed in the Rad51 bound DNA filaments used to initiate recombination through use of chemical probes and by coupling X-ray crystal structures with electron microscopy reconstructions. 3) To test the hypothesis that the Rad51 molecular surfaces that bind multiple protein interactors are conformationally-regulated in the context of specific damage repair steps by elucidating structures of Rad51:protein complexes in the context of ATP/ADP. The long term goal is to characterize Rad51's role in protection against diseases associated with genomic instabilities, such as cancer.